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<html lang="zh-CN">
<head>
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    <title>Document</title>
<style>
    *{
        margin: 0;
        padding: 0;
    }
    .box{
        width: 300px;
        margin: 0 auto;
    }
    ul{
        list-style: none;
    }
    .box>li{
        float: left;
        width: 148px;
        text-align: center;
        border: 1px solid blue;
        background-color:blue;
        color: white;
    }
    .item ul{
        display: none;
        background-color:white ;
        color: black;
    }
    .item:hover ul{
        display: block;
    }
    .box>li li:hover{
        color: blue;
    }
</style>
</head>
<body>
    <ul class="box">
        <li class="item">视频
            <ul>
                <li>全部视频</li>
                <li>java视频</li>
                <li>python视频</li>
                <li>html视频</li>
            </ul>
        </li>
        <li class="item">教程
            <ul>
                <li>pmp</li>
                <li>红帽</li>
            </ul>
        </li>
    </ul>
</body>
</html>